Why can one gas discharge tube (GDT) have a DC sparkover voltage and a substantially higher impulse sparkover voltage? The values are not conflicting ratings. They describe ignition under different voltage rise rates. A DC sparkover test raises voltage relatively slowly, while an impulse test reaches high voltage within microseconds. Because formation of a conductive plasma path takes finite time, the terminal voltage can continue rising during a fast transient before the GDT switches into conduction.
Start with the test condition attached to each value
DC sparkover voltage is measured with a specified slow voltage ramp. Many GDT series use 100 V/s, but that condition must not be assumed for every product. The result may be stated as a nominal value with percentage tolerance or as minimum, nominal, and maximum limits. It is a reference for ignition under a slowly increasing voltage, not a universal ceiling for every transient waveform.
Impulse sparkover voltage is measured at a much faster voltage rate of rise, commonly 100 V/µs or 1 kV/µs in product tables. It is often specified as a maximum value. The rate of rise, polarity, sample conditioning, temperature, and statistical criterion belong to the specification. A statement such as “700 V impulse sparkover” is incomplete if the associated voltage slope is omitted.
Why the fast impulse produces a higher sparkover voltage
In its normal state, a GDT presents high insulation resistance and very low capacitance. As voltage rises, the gas between its electrodes begins to ionize. The discharge develops through initial breakdown and glow behavior toward a low-voltage, high-current arc. This is a physical process rather than an ideal zero-delay switch.
Initial charge carriers, gas composition and pressure, electrode geometry, temperature, previous discharge history, and the applied voltage slope can all affect the ignition event. A slow ramp provides more time for the discharge path to develop, so sparkover occurs around the specified DC range. During a steep impulse, the applied voltage can keep rising while the ionization path is still forming. The resulting dynamic overshoot is why the maximum impulse sparkover voltage is normally above the nominal DC sparkover voltage.
A higher rate of rise will often correspond to a higher specified impulse limit, but there is no universal conversion factor. The difference must be read from the same part number under the stated test conditions. Applying a ratio taken from one family to another removes the effects of geometry, gas system, manufacturing limits, and test method.
How to read a concrete datasheet example
The Bourns GDT25-60 product brief separates the conditions clearly. At 100 V/s, it lists a 480 V minimum, 600 V nominal, and 720 V maximum DC breakdown voltage. The maximum impulse breakdown voltage is 1000 V at 100 V/µs and 1100 V at 1000 V/µs. This example demonstrates the effect of voltage slope, but its numbers apply only to that series. They do not establish a general rule for every 600 V GDT.
The same table also specifies insulation resistance, capacitance, AC discharge current, and impulse discharge current under stated waveforms and operation counts. These parameters answer different questions. Sparkover voltage describes when the device ignites; discharge-current ratings describe what it can conduct after ignition. Neither one proves that the GDT will extinguish safely when connected to a continuously energized source.
Build a protection window with two boundaries
The lower boundary comes from the highest voltage that may appear during normal operation. Include steady tolerance, ripple, common-mode offset, polarity, and any permitted temporary condition. Those voltages should not cause nuisance ignition. Insulation resistance, DC holdover behavior, and the applicable non-follow-current condition may also matter. Merely keeping the operating voltage below the nominal DC sparkover value is not a complete immunity check.
The upper boundary comes from the withstand capability of the protected equipment. For a fast event, use the maximum impulse sparkover voltage at a rate of rise that represents the threat. Then include GDT production limits, generator source impedance, wiring inductance, the position of the measurement, and any upstream or downstream protective components. If the protected circuit cannot tolerate the specified impulse sparkover maximum, the nominal DC value does not close the protection gap.
A coordinated GDT plus MOV, TVS diode, or other limiting stage may reduce let-through voltage. That combination cannot be assessed by simply adding two table values or choosing the lower rating. Distance between stages, decoupling or series impedance, dynamic response, current and energy sharing, and the safe state after a fault must be verified in the assembled circuit.
Sparkover is only the start of the event
After ignition, a GDT can transition to a low arc voltage while carrying a high surge current. This crowbar behavior gives the device its high-current capability, but it can also allow the power source to supply follow current after the original transient has ended. Available fault current, source impedance, AC zero crossings, DC supply voltage, and holdover characteristics determine whether the device returns to its insulating state.
Review DC holdover voltage, arc voltage, AC discharge-current capability, impulse waveform and operation count, life drift, and any fail-short mechanism for the selected series. Coordinate these characteristics with fusing, current limiting, or disconnection where required. A lower impulse sparkover voltage is not automatically safer if the resulting arc cannot be extinguished or if the discharge duty exceeds the device rating.
A practical datasheet and validation workflow
- Define maximum continuous voltage, ripple, polarity, source impedance, and permitted temporary overvoltage.
- Read the DC sparkover ramp rate and all minimum, nominal, maximum, or tolerance limits.
- Match the transient voltage slope to the appropriate maximum impulse sparkover specification.
- Check discharge-current waveform, peak, polarity, operation count, interval, and end-of-life limits.
- Assess arc voltage, holdover or follow current, and whether the energized source can extinguish the discharge.
- Model wiring parasitics and coordinated stages, then validate voltage and current at defined measurement points.
The test record should preserve generator open-circuit voltage, short-circuit current, source impedance, waveform, polarity, sample preconditioning, ambient condition, and pass criteria. A note that a GDT “passed a surge level” without those conditions cannot be reproduced or safely transferred to another circuit.
Common interpretation errors
Do not treat DC sparkover as the highest voltage the protected load will ever see. Do not compare two impulse values without matching their V/s or V/µs conditions. Do not confuse maximum impulse sparkover voltage with the much lower arc voltage after conduction begins. Finally, do not use a surge-current rating as proof of follow-current interruption or complete equipment compliance.
FAQ
Can DC sparkover voltage be used as the maximum clamping voltage?
No. For a fast transient, use the maximum impulse sparkover voltage at the relevant rate of rise and include interconnect and coordination effects. A GDT does not regulate its terminal voltage at the DC sparkover value.
Does a faster impulse always make a GDT respond later?
Dynamic overshoot commonly increases with voltage slope, but device construction, conditioning, and test method also matter. Use the manufacturer’s specified maximum at the defined rate rather than assuming a fixed delay or multiplier.
Are two GDTs interchangeable if their nominal DC sparkover values match?
No. Compare impulse sparkover limits, discharge-current waveforms and operation counts, capacitance, insulation resistance, holdover and follow-current behavior, life drift, package, environmental limits, and approvals.
Why does the measured equipment voltage exceed the datasheet impulse value?
First confirm that the applied slope, polarity, source impedance, and sample condition match the datasheet. Then separate GDT terminal voltage from voltage added by leads, PCB traces, coordinated stages, and the probing loop. Measurement bandwidth and placement can materially change the observed peak.
Practical conclusion
DC sparkover voltage describes GDT ignition during a specified slow ramp. Impulse sparkover voltage describes the dynamic maximum under a specified fast voltage slope. A reliable design must keep normal operation below the non-ignition boundary while holding the worst transient below the protected circuit’s withstand limit. It must then verify discharge current, follow current, extinction, life, and layout. Selecting only by nominal DC sparkover can underestimate fast-transient let-through voltage and overlook what happens after the GDT fires.
Continue with IEC 61000-4-5 Surge Test Explained, Protection Circuits, MOV vs TVS Diode, and the UBAEC contact page.
